Proliferation and differentiation of cells of multicellular organisms are controlled by hormones and polypeptide growth factors. These diffusable molecules allow cells to communicate with each other and act in concert to form organs and repair damaged tissue. Examples of hormones and growth factors include the steroid hormones (e.g. estrogen, testosterone), parathyroid hormone, follicle stimulating hormone, the interleukins, platelet derived growth factor (PDGF), epidermal growth factor (EGF), granulocyte-macrophage colony stimulating factor (GM-CSF), erythropoietin (EPO) and calcitonin.
Hormones and growth factors influence cellular metabolism by binding to receptors. Receptors may be integral membrane proteins that are linked to signalling pathways within the cell, such as second messenger systems. Other classes of receptors are soluble molecules, such as the transcription factors.
The study of receptor-ligand interactions has revealed a great deal of information about how cells respond to external stimuli. This knowledge has led to the development of therapeutically important compounds, such as erythropoietin, the colony stimulating factors and PDGF. Nevertheless, many molecules that control cell growth and development are probably yet to be discovered. In some instances, receptors are known, but their ligands remain to be identified. Discovery of new hormones and growth factors would be aided by the provision of new molecular tools, including receptors, ligands and growth factor-dependent cells. The present invention provides such tools as well as other, related advantages.